Add smbios element to schema and configuration to HVM os

Add smbios element to schema and configuration to HVM os

the element has a mode attribute allowing only 3 values:
  - emulate: use the smbios emulation from the hypervisor
  - host: try to use the smbios values from the node
  - sysinfo: grab the values from the <sysinfo> fields

* docs/schemas/domain.rng: extend the schemas
* src/conf/domain_conf.h: add the flag to the domain config
* src/conf/domain_conf.h: parse and serialize the smbios if present